"Tanning" is a process for treating animal hides which employs metal salts or substances found in plants and called tannin or a combination of metal salts and tannin. Tannin and metal tanning salts cause changes in the skins which make the skins resistant to decomposition; flexible; and very strong, which greatly improves their wear resistance. The tanned hides are called leather.
Tanning processes have been employed for many centuries to dress and preserve animal hides or skins. The treated skins are widely used for footwear, clothing, upholstery, sports equipment, saddlery, and many other items.
In conventional tanning processes, freshly split animal hides or skins are initially cured--typically by salting and/or drying--to prevent decay. These skins are treated to remove loose flesh and excess hair, typically by soaking them in a water bath containing enzymes and sometimes other chemical additives. The pretreated skins, commonly referred to as pelts, are bated by soaking them in an appropriate solution formulated to soften the pelts, to further remove excess proteins, and to improve color. In some cases, fatty substances are also leached out by additional soaking in solvent and/or detergent based solutions. The pelts are then chemically treated in vats with selected tanning agents of vegetable or mineral origin for conversion into leather. The leather is frequently treated with oils or fats or other lubricating agents to obtain soft and supple characteristics; and this is followed by appropriate dyeing, drying, and finishing steps.
The cleaning of such conventionally produced leathers can be difficult or impossible because leathers made by conventional tanning processes do not stay soft and compliant when wetted with water or water-based solutions of the type used for washing traditional woven fabrics. Instead, when wetted, leather products exhibit a tendency to become stiff and/or brittle as they dry with cracks or splits often occurring. Moreover, permanent discoloration and/or stains can occur when a conventional leather is wetted with water. As a result, cleaning techniques for leather have generally been limited to relatively costly dry cleaning processes employing organic solvents.
Attempts have been made to permit at least some contact of leathers with water without significant adverse consequences. For example, there are specially treated suede leathers somewhat capable of withstanding occasional spot cleaning with water-based solutions with only minimal change to surface characteristics or color. Other leathers have been treated during post-tanning processes with specialized oil formulations to enhance their resistance to water penetration and damage. However, even these leather products are not capable of withstanding repeated washing in water in the manner used for cleaning traditional woven fabrics.
U.S. Pat. No. 4,999,024 discloses a novel tanning process capable of producing leathers which, in contrast to even those made by the specialized treatment of conventionally tanned leathers, can be washed in water without cracking, drying out, or otherwise deteriorating. Instead, the leathers made by the patented process are soft, supple, and compliant and retain these characteristics, even after repeated washing.
The process disclosed in the '024 patent employs a preliminary step in which hides are impregnated with a grease/oil lubricating solution. the impregnated skins are then preserved by tanning to produce a soft and supple leather. Residual lubricating solution in the pores of the leather keeps the leather supple even after repeated washing in water.
Optimum results are obtained by starting with relatively thick skins having a well developed pore structure. The skins are typically split and cured by salting and/or drying. These skins are subjected to a preliminary cleaning step including tumbling in a bath of cold water. The tumbled skins are placed in a water-based bath containing a grease/oil lubricating solution designed to penetrate and impregnate the pores of the skins. The skins are tumbled in the lubricating solution at an elevated temperature, causing the pore structure of the skin to swell and absorb substantial quantities of the lubricating solution. Optimum results are obtained by first tumbling the skins in the heated bath and then allowing the skins to stand in the bath for at least several hours.
The thus-treated skins are removed from the lubricating solution bath, drained, and bated by tumbling them in a mild acid salt solution. Appropriate enzymes are normally added to the bating medium to assist in softening the skins and in removing excess proteins.
The bated skins are rinsed and tanned, preferably with a sequence of mineral and vegetable tanning agents. The result is leathers having a substantially closed pore structure with the absorbed lubricating solution trapped in the pores.
After the last (typically vegetable) tanning step, the skins are tumbled again in a water-based grease/oil lubricating solution containing additional lubricating agents to ensure that the leather is soft and supple. The resultant leather is drained and then finished in a conventional manner.
U.S. Pat. No. 4,999,024 is hereby incorporated by reference in and made a part of this specification.